Objects of aluminum and alloys of aluminum having colored coatings and process

ABSTRACT

The process for providing colored coatings on aluminum and alloys of aluminum in which the surface is first anodized to form a porous layer and colored particles of metal compound are deposited in the porous layer by alternating current electrolysis with the object mounted as an electrode in an electrolyte containing the salt of the metal, and then subjecting the deposited particles to dissolution by mounting the object as an anode for direct or rectified current electrolysis until a color of the desired value is secured.

0 United States Patent [151 3,664,932 Patrie 5] May 23, 1972 [54]OBJECTS OF ALUMINUM AND ALLOYS 1 Referenms Cited OF ALUMINUM HAVINGCOLORED UNITED STATES PATENTS COATINGS AND PROCESS 3,382,160 5/1968Asada .104/35 N [72] Inventor: Jos Patrie, Grenoble, France PrimaryExaminer john H. Mack [73] Assignee: Cegedur GP, Paris, France AssistantExaminer-Regan J. Fay [22] Filed: Dec. 3, 1969 Att0rneyMcDougall, Hersh& Scott [2]] Appl. No.: 881,846 [57] ABSTRACT The process for providingcolored coatings on aluminum and [30] Foreign Application Priority Dataalloys of aluminum in which the surface is first anodized to form aporous layer and colored particles of metal compound DEC. 6, 1968 Franceare deposited in the porous layer y alternating current elec trolysiswith the object mounted as an electrode in an elec- [52] U.S. Cl...204/35 N, 294/58 trolyte containing the salt of the metal, and thensubjecting the [51 II!!- Cl. ..C23 5/50 deposited particles todissolution by mounting the object as an [58] Fleld 0f Search ..204/35N, 38, 58, 42 anode for direct or rectified current electrolysis until acolor of the desired value is secured.

18 Claims, No Drawings OBJECTS OF ALUMINUM AND ALLOYS OF ALUMINUM HAVINGCOLORED COATINGS AND PROCESS This invention relates to aluminum andalloys of aluminum having a colored protective coating and methods forpreparation of same.

Numerous processes are available for use in producing a colored coatingon the surface of anodized aluminum and alloys thereof. In the FrenchPat. No. 880,095, description is made of a coloring process comprisingdepositing particles of metal compound into the pores of a layer formedby anodization and while under alternating current electrolysis. Thelayer that is obtained is dark in color, depending somewhat upon thetime of alternating current electrolysis. As a result, it has been thepractice to terminate electrolysis when the desired color has beenobtained and then seal the layer in boiling water. This process issubject to a serious disadvantage in that, at the start of theelectrolysis, the deposit is non-uniform with the result that anon-uniform color is obtained.

It is an object of this invention to produce and to provide a processfor producing a highly uniform clear color on an anodized layer ofaluminum and in which the color is capable of being reproduced.

The process of this invention comprises restoring, by anodicdissolution, the clear colors observed during the initial stages ofdeposition, after a dark or black color effect has been obtained on theobject formed of aluminum or alloys thereof, due to the presence ofmetal or metal compound particles in the pores of the formed layer ofalumina. During anodic dissolution, the color turns clear as thetreatment progresses and, contrary to what is observed in electrolyticcoloring, the color rapidly becomes uniform over the entire surface ofthe object. In addition, the anodic dissolution treatment is highlyflexible and thus gives more latitude for regulating the productionparameters. This is highly advantageous insofar as commercial operationis concerned.

The following will briefly describe the practice of this invention forproducing a colored layer on aluminum or alloy of aluminum. For thispurpose, the object is initially anodized, such as in a sulphuric acidbath, to provide the object with a porous layer of alumina. Coloredparticles of a metal or metal compound are then deposited byelectrolysis into the pores of the anodized layer, using a bathcontaining a dissolved salt of the constituent metal of the particles,giving a dark color effect. The electrolysis is operated withalternating current with the object to be coated forming one of theelectrodes, while the other electrode, referred to as thecounter-electrode, is made either of the constituent metal of the saltdissolved in the electrolyte or of a material which is resistant to theeffect of the bath and which does not give rise to any secondaryreactions during the electrolysis, such for example as stainless steel.

This is followed by anodic dissolution, in accordance with the practiceof this invention. The object to be coated, which has a dark color oreven a black color due to the presence of an excess of colored particlesin the pores of the alumina layer, is mounted as an anode in acontinuous current or direct current electrolysis cell. It is believedthat the anion of the electrolyte combines with the metal compounddeposited in the pores of the alumina layer to form a soluble salt ofthe metal. Anodic dissolution is accompanied by a lightening andstandardization of the color due to the decrease and probably to thestandardization of the thickness of the deposit. The operation isterminated by cutting the current when the required color has beenobtained.

In a preferred embodiment of the invention, the dissolution stage iscarried out in the same bath and in the same cell as the deposition byelectrolysis. It is sufficient, for this purpose, to provide a switchingdevice by which it is possible to change from alternating current todirect current. More simply, it is sufficient to provide a diode or adiode bridge in the circuit for converting the alternating current intoa rectified unfiltered current which gives the same results as thedirect current.

The electrolyte comprises an acid solution, and preferably a sulphuricacid solution, of a salt of a metal such as copper,

nickel, cadmium, cobalt, etc. The concentration of metal salts dissolvedin the electrolyte may range from 0.15 g/liter to an amount to provide asaturated solution. The operation is carried out at bath temperaturewithin the range of 5-30 C and preferably of about room temperature,either at a constant current density of less than 5 A/dm and preferablyat a current density within the range of 0.1 to 2 A/dm or at a constantvoltage within the range of 10 to 50 volts and preferably 15 to 25 voltsfor a period of from 10 seconds to 30 minutes, and generally from A tol5 minutes. While it is preferred to carry out anodic dissolution in thesame electrolyte as for deposition by electrolysis, preferably under thesame temperature conditions, use can be made of a different bath of abuffered sulphuric acid or salt thereof of corresponding pH.

The final colored layer can be fixed by any conventional process, suchas by sealing in boiling water, varnishing or the like.

The color obtained is governed by the nature of the metal and the colorintensity is governed both by the current density and by the duration ofthe anodic dissolution for the same initial color effect. For example, awide range of reds from pink to deep scarlet, including copper shades,can be obtained with an electrolyte containing copper salts. The saltsof nickel, cadmium or cobalt enable browns or gold hues to be obtained.

The final appearance is somewhat related to the surface quality of theobject before the porous layer of alumina is formed. For example, on abrilliant or glossy surface, the color effects obtained by the processare brilliant while on a dull or mat surface, a dull or mat color willbe secured. Accordingly, it is advantageous to modify the surfacecharacteristics of the object before coating. Such surface changes canbe effected by conventional or known means including polishing, buffing,glazing, and the like, depending upon the finish required. For example,a mat finish is obtained by cleaning the object for 10 minutes at 50 Cin a soda solution having a concentration of 50 g/liter, after which itis washed in a stream of water followed by immersion in nitric acid (36Be) and then by further rinsing in a stream of water. The glossy orshiny finish is obtained by mechanical polishing or by chemical orelectrolytic glossing. I

The following examples are given by way of illustration, but not by wayof limitation, of the practice of this invention.

In the following examples the objects are subjected to the followinggeneral steps:

surface preparation by one of the processes referred to above;

anodization in a sulphuric acid bath;

coloring with alternating current'in acid solution containing salts ofmetals of copper, nickel, cobalt and the like, with counter-electrodesmade with the constituent metal of the salts or of stainless steel, atcurrent densities within the range of 0.5 to 5 A/dm for a period of timewithin the range of l to 15 minutes, to obtain black, red or very deepbronze color effects; and

partial anodic dissolution in the coloring bath.

It is preferred, though not essentially a part of the invention, to sealthe coating as by 30 minutes immersion in distilled boiling watercontaining from 0.5 to 2 g/liter of nickel acetate.

EXAMPLE 1 A 99.5 percent pure aluminum sheet is mechanically treated toprovide a satin surface and then anodized in a sulphuric acid bath togive a porous surface layer of alumina having a thickness of 15 microns.It is then colored by mounting as an electrode in an electrolysis cellfed with alternating current in which the electrolyte is a sulphuricacid solution of copper sulphate having the following composition:

CuSO 5H O,: 20 g/liter H 50 5 g/liter The counter-electrode is made ofstainless steel.

After 5 minutes of treatment at 0.6 A/dm a sheet which is dark red incolor is obtained.

The cell is then converted to direct current with the sheet mounted asthe anode. The sheet is removed after 3 minutes of treatment at 0.2 A/dmat about 20 C.

After scaling in boiling water, a sheet having a clear copper red coloris obtained. The color effect is uniform and very dull in appearance andis resistant both to ultra-violet light and to corrosion.

EXAMPLE 2 An aluminum sheet containing 0.6 percent magnesium ischemically treated to provide a shiny finish and then anodized in asulphuric acid bath to obtain a porous layer of alumina having athickness of 20 microns. It is then mounted as an electrode in anelectrolysis cell fed with alternating current in which the electrolyteis a sulphuric acid solution of copper sulphate having the followingcomposition:

CuSO, SH O: 20 g/liter H 80 g/liter The counter-electrode is made ofcopper.

A sheet which is black in color is produced after minutes of treatmentat 0.6 A/dm THe cell is then fed with rectified direct current(unfiltered) with the sheet mounted as the anode. The sheet is removedafter 1 minute and 30 seconds treatment at 0.4 A/dm at C.

A sheet which is bright red in color is produced and sealed in boilingwater. The color is uniform and brilliant in appearance and resistant toultra-violet rays and to corrosion.

EXAMPLE 3 A section of aluminum alloy containing 0.5% silicon and 0.5%magnesium is mechanically polished and then anodized in a sulphuric bathto provide a surface porous layer of alumina having a thickness of 18microns. It is then colored by mounting as an electrode in anelectrolysis cell fed with alternating current in which the electrolyteis a solution of a mixture of boric acid, nickel sulphate and ammoniumsulphate:

nickel sulphate lOO g/liter boric acid g/liter ammonium sulphate g/literThe counter-electrode is made of nickel.

A section which is dark bronze in color is produced after 5 minutesoftreatment at 0.4 A/dm.

The section is mounted as the anode and the cell is fed with directcurrent. The section is removed after 6 minutes oftreatment at 0.1 A/dmat 25 C.

After sealing in boiling water, the section has a clear, uniform andbrilliant bronze colored finish which is resistant to ultra-violet raysand to corrosion.

EXAMPLE 4 An aluminum alloy section containing 1% by weight silicon, 1%by weight magnesium and 1% by weight manganese is cleaned with soda andthen anodized in a sulphuric bath to obtain a porous surface layer ofalumina having a thickness of 12 microns. It is then colored by mountingas an electrode in an electrolysis cell fed with alternating current, inwhich the electrolyte is a solution of a mixture of boric acid, cobaltsulphate and ammonium sulphate:

cobalt sulphate 20 g/liter boric acid: 25 g/liter ammonium sulphate 15g/liter The counter-electrode is made of stainless steel.

After 10 minutes of treatment at 0.3 A/dm the section takes on a darkbronze color.

The cell is then fed with continuous current or direct cur rent with thesection mounted as the anode. The section is removed after 4 minutesoftreatment at 0.2 A/dm at 20 C.

After scaling in boiling water, the section has a clear, uniform matbronze color which is resistant to ultra-violet rays and to corrosion.

EXAMPLE 5 An aluminum alloy section containing 1% by weight silicon, 1%by weight magnesium and 1% by weight manganese is mechanically polishedand then anodized in a sulphuric acid bath to provide a porous surfacelayer of alumina having a thickness of 15 microns. It is then colored bymounting as an electrode in an electrolysis cell fed with alternatingcurrent. in which the electrolyte is a sulphuric acid solution of coppersulphate:

CuSO,- SH O 20 g/liter H SO 5 g/liter The counter-electrode is made ofcopper.

After 10 minutes of treatment at 0.6 A/dm", the section is black incolor.

The cell is then fed with direct current with the section mounted as ananode. The section is removed after 2 minutes of treatment at 20 voltsat 20 C.

After sealing in boiling water, the section has a uniform and brilliantcopper red finish which is resistant to ultra-violet rays and tocorrosion.

It will be apparent from the foregoing that there is provided a new andimproved process for producing colored coatings on aluminum and alloysof aluminum in which the colored coating is permanent, resistant toultra-violet rays, resistant to corrosion, and in which the coloredsurface is uniform throughout.

It will be understood that changes may be made in the details offormulation and operation without departing from the spirit of theinvention, especially as defined in the following claims.

I claim:

1. In a process for color coating objects of aluminum and alloys ofaluminum, the steps of anodizing the surface of the object to be coloredto produce a porous alumina layer, mounting the object as an electrodein an electrolytic bath containing in solution a salt of a metal whichdeposits colored particles in the alumina layer in response toalternating current, and passing an alternating current through the bathto deposit colored particles of the metal or a compound of the metal inthe porous coating, and then mounting the object as an anode in anelectrolytic bath while passing a current therethrough for dissolutionof colored particles until the desired color is obtained.

2. The process as claimed in claim 1 which includes the step ofsealingthe colored coating that is formed.

3, The process as claimed in claim 2 in which the colored coating issealed with boiling water.

4. The process as claimed in claim 1 in which the current fordissolution is direct current.

5. The process as claimed in claim 1 in which the current fordissolution is rectified unfiltered current.

6. The process as claimed in claim 1 in which the electrolytic bath forthe alternating current electrolysis is the same as the electrolyticbath for dissolution so that the object remains in the same bath forboth direct current electrolysis and dissolution electrolysis.

7. The process as claimed in claim 6 in which the change fromalternating current to direct or rectified current is effected by merelyswitching from one to the other.

8. The process as claimed in claim 1 which includes the step ofsubjecting the object to be colored to a surface treatment prior toelectrolysis treatment to modify the appearance of the formed coloredcoating.

9. The process as claimed in claim 1 in which the metal compounddeposited in the porous layer contains a compound of copper to produce acolor within the range of pink to copper red to bright red. '7

10. The process as claimed in claim 1 in which the metal compounddeposited in the porous layer contains a compound of cobalt to produce aclear bronze color effect in the colored coating.

11. The process as claimed in claim 1 in which the metal 15. The processas claimed in claim 1 in which the current compound deposited in theporous layer contains a compound for dissolution is within the range ofto 25 volts. of nickel to produce a uniform clear bronze color effect inthe 16. The process as claimed in claim 1 in which the dissolucoloredcoating. tion is carried out for a time within the range of U6 to 3O 12.The process as claimed in claim 1 in which the current 5 minllmsfdissolution has a Gun-em dcnsity [cs5 than 5 17. The process as claimedin claim 1 in which the dissolution is carried out for a time within therange of 1/2 to 15 13. The process as claimed in claim 1 in which thecurrent for dissolution has a current density within the range of 0.1 to2 A/dm 14. The process as claimed in claim I in which the current fordissolution is within the range of 10 to 50 volts.

minutes 18. An object of aluminum or alloy of aluminum having a

2. The process as claimed in claim 1 which includes the step of sealingthe colored coating that is formed.
 3. The process as claimed in claim 2in which the colored coating is sealed with boiling water.
 4. Theprocess as claimed in claim 1 in which the current for dissolution isdirect current.
 5. The process as claimed in claim 1 in which thecurrent for dissolution is rectified unfiltered current.
 6. The processas claimed in claim 1 in which the electrolytic bath for thE alternatingcurrent electrolysis is the same as the electrolytic bath fordissolution so that the object remains in the same bath for both directcurrent electrolysis and dissolution electrolysis.
 7. The process asclaimed in claim 6 in which the change from alternating current todirect or rectified current is effected by merely switching from one tothe other.
 8. The process as claimed in claim 1 which includes the stepof subjecting the object to be colored to a surface treatment prior toelectrolysis treatment to modify the appearance of the formed coloredcoating.
 9. The process as claimed in claim 1 in which the metalcompound deposited in the porous layer contains a compound of copper toproduce a color within the range of pink to copper red to bright red.10. The process as claimed in claim 1 in which the metal compounddeposited in the porous layer contains a compound of cobalt to produce aclear bronze color effect in the colored coating.
 11. The process asclaimed in claim 1 in which the metal compound deposited in the porouslayer contains a compound of nickel to produce a uniform clear bronzecolor effect in the colored coating.
 12. The process as claimed in claim1 in which the current for dissolution has a current density less than 5A/dm2.
 13. The process as claimed in claim 1 in which the current fordissolution has a current density within the range of 0.1 to 2 A/dm2.14. The process as claimed in claim 1 in which the current fordissolution is within the range of 10 to 50 volts.
 15. The process asclaimed in claim 1 in which the current for dissolution is within therange of 15 to 25 volts.
 16. The process as claimed in claim 1 in whichthe dissolution is carried out for a time within the range of 1/6 to 30minutes.
 17. The process as claimed in claim 1 in which the dissolutionis carried out for a time within the range of 1/2 to 15 minutes.
 18. Anobject of aluminum or alloy of aluminum having a uniform colored coatingproduced by the method of claim 1.